Recent improvements in microparticle synthesis and handling have prompted new research into the engineering and fabrication of single and multilayered microspheres through traditional physical and chemical vapor depositions. At the University of Delaware, we have developed a custom batch coating process utilizing a vibro-fluidized mixing vessel to deposit thin-films onto the surface of microparticle substrates through R.F. magnetron sputtering. This process opens up a number of design possibilities for single and multilayered microsphere technologies that can be used to improve the optical performance of several optical filtering applications. Through the use of custom design and simulation software, we have optimized a number of filter designs and validated these findings through commercial software. Specifically, we have aimed to improve upon the mass extinction performance seen by traditional materials in the long wave infrared spectrum (LWIR, λ=8-12μm). In order to do this, we have run a series of experiments aimed at creating ultra-lightweight metallic hollow-spheres. Aluminum thin-films have been successfully deposited onto a number of substrates including hollow glass microspheres, high density polyethylene microspheres, and polystyrene foam spheres. By depositing the thin-films onto polymer substrates we have been able to remove the solid core after deposition through a thermal decomposition or chemical dissolution process, in an effort to reduce particle mass and improve mass extinction performance of the filter. A quantum cascade laser measurement system has been used to characterize the optical response of these fabricated aluminum hollow-spheres and have largely agreed with the expected simulated results.
Silicon slot waveguides leverage the field enhancement provided by the continuity of normal electric flux density across a dielectric boundary to confine an optical mode to a void between two proximal silicon strips. Silicon-organic hybrid slot modulators make use of this mode profile by infiltrating the slot region with a non-linear organic electro-optic material (OEOM) for modulation. The dual slot modulator takes this idea a step further by similarly confining a propagating RF mode to the same slot region to increase modal overlap for improved modulation efficiency. This effect is achieved by aligning a titanium dioxide RF slot along a conventional silicon slot waveguide. The TiO<sub>2</sub> has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. As a result of the large modal overlap and high electro-optic activity of the OEOM this design can produce measured phase modulated V<sub>π</sub>L of less than 1.40 V•cm. Furthermore, as the modulator operates without the introduction of a doping scheme it can potentially realize high operational bandwidth and low loss. We present work towards achieving various working prototypes of the proposed device and progress towards high frequency operation.
Dual vertical slot modulators leverage the field enhancement provided by the continuity of the normal electric flux density across a boundary between two dielectrics to increase modal confinement and overlap for the propagating optical and RF waves. This effect is achieved by aligning a conventional silicon-based optical slot waveguide with a titanium dioxide RF slot. The TiO<sub>2</sub> has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. The dual slot design confines both the optical and RF modes to the same void between the silicon ribs of the optical slot waveguide. To obtain modulation of the optical signal, the void is filled with an organic electro optic material (OEOM), which offers a high optical non-linearity. The optical and RF refractive index of the OEOM is lower than silicon and can be deposited through spin processing. This design causes an extremely large mode overlap between the optical field and the RF field within the non-linear OEOM material which can result in a device with a low Vπ and a high operational bandwidth. We present work towards achieving various prototypes of the proposed device, and we discuss the fabrication challenges inherent to its design.
Low driving voltage and high-speed electro-optic (EO) modulators are of great interest due to
their wide variety of applications including broadband communication, RF-photonic links,
millimeter wave imaging and phased-array radars. In this paper we propose symmetric design,
analysis, and optimization of a novel, high speed and ultra-low driving voltage traveling wave
EO modulator based on a dual RF-photonic slot waveguide. Preliminary simulation results
demonstrate the DC electro-optic response and half-wavelength voltage-production Vπ-<i>L</i> of
0.1~0.2 V•cm can be achieved for this design. The electro-optic response demonstrates the
proposed device is capable of ultra-high speed operation that covers entire RF spectrum.
Wavelength-division multiplexing (WDM) is the transmission of many signals through a single communication channel
using different wavelengths, each of which carries a separate, independent signal. We present and discuss a
reconfigurable WDM based on slow-light, functioning as a bi-directional optical routing and processing network,
consisting of photonic crystals designed as drop/add filters. The photonic crystal based routing elements consist of two
waveguides coupled through a resonant cavity. Photonic crystals offer the ability to achieve separation of many channels
on a much smaller scale than their predecessors. Photonic crystals have led a challenging frontier of miniaturization and
large scale integration of high-density optical interconnects, and with the aid of nanomembranes, optical routing
networks can set a new standard for high-density optical interconnects.
There is a growing need for miniature low-cost chemical sensors in monitoring environmental conditions.
Applications range from environmental pollution monitoring, industrial process control and homeland security threat
detection to biomedical diagnostics. Integrated opto-electronic sensors can provide chemical & biological sensing by
monitoring attachment induced changes in the refractive, absorptive, or luminescent properties of materials.
Nanomembranes (NMs) are single crystals that have been released from SOI substrates and redeposited on foreign
flexible or flat substrates enabling the best features of different materials. Silicon Nanomembrane technology can enable
the fabrication of compact, replaceable/disposable and highly sensitive optoelectronic sensors for chemical and
In this paper, we present novel designs for the realization of organic-inorganic hybrid material systems and
develop concepts and designs for silicon-organic hybrid ultrafast RF Photonic Devices. The designs
presented combine, crystalline electro-optic materials, conventional crystalline materials, and amorphous
polymers. Numerical simulation results as well as fabrication results are also included.
We present and discuss several of the benefits associated with using chip-scale optical interconnects in reconfigurable
computing systems. As is well known, by removing metallic traces in high-speed systems, many signal integrity issues
are reduced, or eliminated, e.g., parasitic capacitance and inductance associated self-induced affects and trace overlay.
In addition, photonic systems can require less power and offer higher efficiency, thereby, giving rise to reduced thermal
energy dissipation. However, at least in the case of reconfigurable processors there are several additional advantages. A
case in point is that of field programmable gate arrays (FPGAs), which is a technology that has been plagued by
interconnect limitations. To address this, we have developed an interconnect network that will enable fully
reconfigurable processors, or FPGAs. Our approach is based on a photonic crystal cross-bar switch that enables
complete interconnectivity over large computational-block arrays. Perhaps one of the most attractive benefits of our
approach is that it alleviates the need to perform place and route during processor layout. As such, our approach may
allow for reconfigurable processors consisting of a higher density of computing-blocks along with a faster interconnect
medium. Accordingly, this talk will present numerical studies, design and fabrication of various implementations of
candidate photonic crystal devices for reconfigurable optically interconnected chip-scale networks.
Nanomembranes (NM) are crystalline semiconductor materials (Si, GaAs, SiGe, etc) that have been released from their
substrates and redeposited on foreign, flexible or flat substrates enabling the best features of both materials. Although
they are in fact crystalline in nature and possess the electronic/photonic properties of bulk material, they are flexible,
deformable, and conformable. An obvious choice is silicon-on-insulator (SOI). SOI provides, beyond its application in
the Si industry, the ultimate platform for exploring novel science and technological advancements in this class of
nanomaterial. In SOI, a SiO2 layer is interspersed between a thin crystalline top Si layer and the bottom Si wafer; the
ability to etch this buried oxide selectively creates the nanomembranes. When released from the oxide, this layer can
form extremely flexible strain-engineered thin nanomembranes with thicknesses from several hundred nanometers to
less than 10 nm, and in various shapes. Photonic devices originally structured in an SOI substrate can now be transferred
and stacked on new substrates, rigid and flexible, to increase optical interconnect densities.
Grayscale lithography is an extension of the conventional binary lithographic process for realization of arbitrary three-dimensional features in photoresist materials, with applications especially in micro-optics fabrication. The grayscale photomask possesses a spatially varying transmission that modulates the exposure dose received in the photoresist. By using a low contrast photoresist, such as those based on diazonaphthoquinone (DNQ), the material is only partially removed during development in proportion to the local exposure dose received. In this way, an arbitrary surface topography can be sculpted in the photoresist material. It is common practice in grayscale lithography to encode the transmission levels of the photomask by using the photoresist contrast curve to determine the exposure dose required for a given photoresist thickness at each lateral point in the pattern. This technique is adequate when the surface topography is slowly varying and the photoresist film is thin. However, it is inaccurate when these conditions are not met, because the technique essentially represents a one-dimensional approximation to the lithographic process where the isotropy of the development and the diffractive imaging of the photomask are neglected. Currently we are applying grayscale lithography to the fabrication of a fiber-to-waveguide coupler based on the parabolic reflector, where the efficiency of the device is quite sensitive to fabrication errors in the coupler geometry. In this case the thin photoresist and slowly varying topography conditions are not met, and we turn to more comprehensive process models to determine the appropriate transmission levels to encode in the photomask. We demonstrate that the photomask can be optimized, based on simulation of the lithography process, to produce the required three-dimensional photoresist pattern.
For some time, the micro-optics and photonics fields have relied on fabrication processes and technology borrowed from
the well-established silicon integrated circuit industry. However, new fabrication methodologies must be developed for
greater flexibility in the machining of micro-optic devices. To this end, we have explored grayscale lithography as an
enabler for the realization of such devices. This process delivers the ability to sculpt materials arbitrarily in three
dimensions, thus providing the flexibility to realize optical surfaces to shape, transform, and redirect the propagation of
light efficiently. This has opened the door for new classes of optical devices. As such, we present a fiber-to-waveguide
coupling structure utilizing a smoothly contoured lensing surface in the device layer of a silicon-on insulator (SOI)
wafer, fabricated using grayscale lithography. The structure collects light incident normally to the wafer from a singlemode
optical fiber plugged through the back surface and turns the light into the plane of the device layer, focusing it into
a single-mode waveguide. The basis of operation is total internal reflection, and the device therefore has the potential
advantages of providing a large bandwidth, low polarization sensitivity, high efficiency, and small footprint. The
structure was optimized with a simulated annealing algorithm in conjunction with two-dimensional finite-difference
time-domain (FDTD) simulation accelerated on the graphics processing unit (GPU), and achieves a theoretical efficiency
of approximately seventy percent, including losses due to Fresnel reflection from the oxide/silicon interface. Initial
fabrication results validate the principle of operation. We discuss the grayscale fabrication process as well as the
through-wafer etch for mechanical stabilization and alignment of the optical fiber to the coupling structure. Refinement
of the through-wafer etch process for high etch rate and appropriate sidewall taper are addressed.